U.S. patent application number 13/722262 was filed with the patent office on 2013-07-04 for method and apparatus for pole-slip detection in synchronous generators.
This patent application is currently assigned to ComAp a.s.. The applicant listed for this patent is Tomas Jelen, Milan Kalinka, Miroslav Novak, Roman Taragel. Invention is credited to Tomas Jelen, Milan Kalinka, Miroslav Novak, Roman Taragel.
Application Number | 20130168960 13/722262 |
Document ID | / |
Family ID | 48694232 |
Filed Date | 2013-07-04 |
United States Patent
Application |
20130168960 |
Kind Code |
A1 |
Kalinka; Milan ; et
al. |
July 4, 2013 |
METHOD AND APPARATUS FOR POLE-SLIP DETECTION IN SYNCHRONOUS
GENERATORS
Abstract
A system and method for predicting a pole slip in a synchronous
generator is provided. The system includes a stator voltage
frequency detector to determine the frequency of the stator
voltage, a mechanical frequency detector to determine the
rotational speed of the rotor and a prediction unit that is
operative to disconnect the generator from a power grid if it
determines that that a pole slip is likely based on comparison of
the frequency of the stator voltage and the rotational speed of the
rotor.
Inventors: |
Kalinka; Milan; (Solivar,
SK) ; Novak; Miroslav; (Liberec, CZ) ; Jelen;
Tomas; (Praha, CZ) ; Taragel; Roman; (Poprad,
SK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kalinka; Milan
Novak; Miroslav
Jelen; Tomas
Taragel; Roman |
Solivar
Liberec
Praha
Poprad |
|
SK
CZ
CZ
SK |
|
|
Assignee: |
ComAp a.s.
Prague
CZ
|
Family ID: |
48694232 |
Appl. No.: |
13/722262 |
Filed: |
December 20, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61582715 |
Jan 3, 2012 |
|
|
|
Current U.S.
Class: |
290/7 |
Current CPC
Class: |
H02P 9/006 20130101;
H02P 11/00 20130101; H02J 3/00 20130101; H02P 9/10 20130101; H02H
7/065 20130101 |
Class at
Publication: |
290/7 |
International
Class: |
H02P 11/00 20060101
H02P011/00 |
Claims
1. A system for predicting pole slip in a synchronous generator,
the synchronous generator having a rotor rotating at a rotational
speed that generates a stator voltage in a stator, the system
comprising: a stator voltage frequency detector operative to
determine a frequency of the stator voltage; a mechanical frequency
detector operative to determine the rotational speed of the rotor;
and a prediction unit operatively coupled to the stator voltage
frequency detector and the mechanical frequency detector, the
prediction unit further operative to send a trip signal to a
circuit breaker to disconnect the generator from a power grid when
the prediction unit determines that a probability of a pole slip
condition meets a predetermined value.
2. The system of claim 1, wherein the prediction unit determines
the probability of a pole slip condition by comparing the
difference between the rotational speed of the rotor and the
frequency of the stator voltage.
3. The system of claim 1, wherein the prediction unit determines
the probability of a pole slip condition by calculating a load
angle of the generator and comparing the load angle to the
predetermined value.
4. The system of claim 3, wherein the predetermined value is
between 90 and 110 degrees.
5. The system of claim 1, wherein the mechanical frequency detector
is a count sensor operative to count a number of elements
positioned on the periphery of a prime mover operatively connected
to the rotor.
6. The system of claim 1, wherein the prediction unit determines
the probability of a pole slip condition by calculating a rate of
change of a load angle of the generator and comparing the rate of
change to the predetermined value.
7. The system of claim 3, wherein the prediction unit is further
operative to determine if the rotor has made an extra revolution
and send a trip signal to the circuit breaker to disconnect the
generator from the grid if the extra revolution has occurred.
8. The system of claim 5, wherein the prime mover is an internal
combustion engine.
9. The system of claim 5, wherein the elements positioned on the
periphery of a prime mover are teeth positioned on the periphery of
a gear.
10. A method for predicting pole slip in a synchronous generator,
the synchronous generator having a rotor rotating at a rotational
speed that generates a stator voltage in a stator, the method
comprising: receiving a stator voltage frequency signal indicating
a frequency of the stator voltage; receiving a rotor frequency
signal indicating the rotational characteristics of the rotor;
determining a probability of a pole slip condition based on the
stator voltage frequency signal and the rotor frequency signal; and
sending a trip signal to a circuit breaker to disconnect the
generator from a power grid.
11. The method of claim 10, wherein determining a probability of a
pole slip condition comprises comparing the probability of the pole
slip condition to a predetermined value.
12. The method of claim 10, wherein determining a probability of a
pole slip condition comprises comparing the difference between the
rotational speed of the rotor and the frequency of the stator
voltage.
13. The method of claim 10, wherein determining a probability of a
pole slip condition comprises calculating a load angle of the
generator and comparing the load angle to a predetermined
value.
14. The method of claim 10 wherein the predetermined value is
between 90 and 110 degrees.
15. The method of claim 10, wherein the rotational characteristics
of the rotor include a count of a number of elements positioned on
the periphery of a prime mover operatively connected to the
rotor.
16. The method of claim 10, wherein determining a probability of a
pole slip condition comprises calculating a rate of change of a
load angle of the generator and comparing the rate of change to a
predetermined value.
17. The method of claim 10, further comprising determining if the
rotor has made an extra revolution.
18. The method of claim 15, wherein the number of elements
positioned on the periphery of a prime mover are teeth positioned
on the periphery of a gear.
19. A synchronous generator system comprising: a synchronous
generator having a rotor and a stator, wherein the rotor has
windings and the stator has windings; an excitation current coupled
to the rotor and operative to supply a dc current to the rotor
windings; a stator voltage frequency detector operative to generate
a stator frequency signal representative of a frequency of the
stator output voltages; a mechanical sensor operative to generate a
rotor frequency signal representative of a rotational speed of the
rotor; and a pole slip prediction unit operative to predict a pole
slip based on the stator frequency signal and the signal
representative of the rotational speed of the rotor.
20. An apparatus comprising: a synchronous generator having a rotor
and a stator, wherein the rotor has windings and the stator has
windings; an excitation current coupled to the rotor and operative
to supply a dc current to the rotor windings; a stator voltage
period detector operative to generate a stator voltage period
signal, representative of a period of the stator output voltages; a
count sensor operative to generate a tooth count signal
representative of a number of gear teeth counted over a given
interval of time; and a pole slip prediction unit operative to
predict a pole slip based on the stator voltage period signal and
the tooth count signal.
Description
RELATED APPLICATIONS
[0001] This application is based on and claims priority to
non-provisional Patent Application No. 61/582,715 filed Jan. 3,
2012, the contents of which are herein incorporated by
reference.
FIELD
[0002] The present disclosure generally relates to synchronous
generators, and in particular to rotating magnetic field
synchronous generators.
BACKGROUND
[0003] Power generators with an apparent power rating above
approximately 5 kVA are generally constructed as rotating magnetic
field or revolving field synchronous generators. Such machines have
the field windings wound on the rotating member of the machine
(i.e., the rotor) and the armature wound on the stationary member
(i.e., the stator). A low power, low voltage, dc current is
conventionally fed to the field windings on the rotor using, for
example an excitation circuit also known as a excitation system or
exciter that may include an ac/dc converter, dc battery or other dc
current generator. The dc current is supplied to the field windings
using, for example, a rotary electrical interface such as a set of
slip rings and/or brushes. Alternatively, the excitation circuit
may include a shaft mounted exciter and a diode-bridge mounted on
the rotor, thereby creating an electromagnet on the rotor. The
rotor is turned by a prime mover such as an internal combustion
engine, a steam turbine, water turbine or any other suitable
engine, turbine or machine, thereby creating a rotating magnetic
field (i.e., rotor magnetic field). The rotor magnetic field is
constant in strength and rotates around the machine at the rotation
speed of the rotor. Under normal operation, the magnitude of the
rotor magnetic field is directly proportional to the dc current
that excites the rotor windings (i.e., the field current).
[0004] The stator generally comprises three sets of coils of wire
(i.e., windings) that are embedded into the stator (typically made
of iron). The rotation of the rotor magnetic field induces a
sinusoidal voltage at each coil or winding. The induced sinusoidal
voltages are identical in magnitude and frequency but shifted 120
degrees with respect to each other. The three coils are distributed
120 degrees apart on the stator in such a manner to obtain three
balanced and sinusoidal voltages having very little harmonic
content to avoid damaging the generator. The magnitude of the
voltage induced into the stator windings is a function of the
intensity of the rotor magnetic field, the rotational speed of the
rotor and the number of turns in the stator windings.
[0005] The frequency of the induced voltages relates to the
rotational speed of the rotor and the number of its poles. When the
generator is coupled to a distribution network (also called grid or
mains), the frequency of the induced stator alternating voltages is
a system parameter for all generators connected to that network. In
a two-pole machine with a 60 Hz output supply current, the speed of
rotation of both the rotor (i.e., the speed of the rotor magnetic
field) and the stator magnetic field will be 60 revolutions per
second or 3600 rpm. The induced voltages in the three phase stator
windings generate their own magnetic field (i.e., the stator
magnetic field). The strength of the stator magnetic field depends
on the current flow in the stator winding.
[0006] When the torque applied to the rotor is zero (i.e., when the
machine is producing no power in the no load state), the magnetic
fields of the rotor and the stator are perfectly aligned. The
instant torque is introduced to the rotor by the prime mover, and
throughout normal operation of the generator, the magnetic fields
in the generator come out of alignment and a small angle between
the magnetic fields is created. This angle is called the load angle
or the torque angle (.beta.). During stable or steady state
conditions, the load angle (.beta.) is generally less than 90-110
degrees.
[0007] During steady state, the load angle creates a force between
the fields opposing the acceleration of the machine and energy flow
from the machine (i.e., the generator) to the system (i.e., the
grid). The rate of energy flow or power output of the machine is
proportional to the strength of the magnetic fields and the sine of
the load angle.
[0008] As the prime mover accelerates, the load angle increases and
the force opposing the rotation increases and the machine speed
stays constant. Generally, if the strengths of either magnetic
fields is increased, the power output of the machine remains
constant, but the increased forces between the fields pull the
rotor back towards its no load position and the load angle
decreases. In other words, increasing the rotor magnetic field
strength decreases the load angle (with power output staying
constant) and increasing the speed of rotation of the rotor (e.g.,
by the prime mover) increases the load angle and the power output
increase.
[0009] The maximum power output for the strength of the magnetic
poles is found when the load angle of the generator is
approximately 90-110 degrees. If the power input to the generator
starts to push the rotor past the 90-110 degree position, the
retarding forces on the rotor start to decrease and the rotor speed
will start to accelerate and travel faster than the rotating
magnetic field of the armature. At this point, the rotor magnetic
flux is starting to slip with respect to the stator magnetic flux.
If the rotor accelerates such that the rotor takes an extra
revolution than the rotating magnetic field (i.e., a load angle of
360 degrees or more), a pole slip has occurred. Immediately before,
during and after the occurrence of a pole slip, the machine will
undergo severe mechanical stresses as magnetic forces apply torques
on the shaft to first try to brake the machine and then accelerate
it. Such braking and acceleration forces often damage the generator
or decrease the life of a generator.
[0010] The foregoing description of the operation of a generator
and the occurrence of a pole slip was explained by reference to a
separate rotor magnetic field and a separate stator magnetic field.
In reality, however, there is only one magnetic field in a
generator. This generator magnetic field is found in the airgap
between the rotor and the stator and can be thought of as the
resultant magnetic field produced by the combination of the rotor
and stator magnetic fields. Mathematically, the resultant or airgap
flux (.PHI.r) is equal to the summation of the flux of the stator
field (.PHI.s) and flux of the rotor field (.PHI.f):
.PHI.r=.PHI.s+.PHI.f
[0011] Because pole slips are capable of causing significant stress
and potential damages to generators, it has become important to
diagnose the occurrence of a pole slip and disconnect the generator
from the grid by removing the dc current supply from the rotor
windings promptly upon such an event. Conventionally, so-called
impedance methods or schemes are currently implemented to detect
the occurrence of a pole slip and shut down the generator to avoid
continued damage to the machine. These impedance methods are
generally complicated and expensive to implement and include the
measurement of both active and reactive power. More importantly,
they are incapable of "predicting" a pole slip. Instead, such
conventional methods simply detect when a pole slip has actually
occurred.
[0012] While large generators (e.g., generators that are capable of
producing 30-200+MW of active power) are generally more likely to
"survive" several pole slips, smaller generator (e.g., generators
that only produce on the order of 1 MW of active power) are less
likely to physically survive a pole slip. As such, a need exists
for an apparatus and method of predicting when a pole slip will
occur instead of merely reacting to the occurrence of a pole
slip.
[0013] In one aspect of the present disclosure, an improved device
for determining the probability or likelihood of a pole slip is
described. The ability to predict a pole slip condition can allow
actions to be taken before a pole slip occurs and prevent the
undesirable consequences of the pole slip.
[0014] In another aspect of the present disclosure, detection
devices are described. The detection devices or sensors collect
information regarding the characteristics of various aspects of an
operating generator and supply this information to other devices
that can predict a pole slip condition.
[0015] In still another aspect of the present disclosure, a
synchronous generator system is described. The synchronous
generator system includes detection devices for collecting
information regarding the synchronous generator and supply this
information to a device that is able to predict a pole slip
condition. The system may further include a circuit breaker or
other device that can disconnect the generator from the power grid
if a pole slip condition is detected.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The disclosure will be more readily understood in view of
the following figures.
[0017] FIG. 1 illustrates a rotating magnetic field synchronous
generator and the magnetic flux associated with the rotor (i.e.,
the rotor or field flux);
[0018] FIG. 2 illustrates a rotating magnetic field synchronous
generator and the magnetic flux associated with the stator (i.e.,
the stator or armature flux);
[0019] FIG. 3 illustrates a rotating magnetic field synchronous
generator and the resultant or airgap flux produced by the
interaction of the rotor flux and the stator flux;
[0020] FIG. 4 illustrates a rotating magnetic field synchronous
generator with the rotor positioned at a load angle of 45
degrees;
[0021] FIG. 5 illustrates a rotating magnetic field synchronous
generator with the rotor positioned at a load angle of 110 degrees,
at the edge of the stability zone;
[0022] FIG. 6 illustrates a rotating magnetic field synchronous
generator with the rotor positioned at a load angle of 270 degrees,
90 degrees from the occurrence of a pole slip;
[0023] FIG. 7 illustrates a block diagram of a prime mover coupled
to a rotating field synchronous generator in accordance with one
embodiment of the present disclosure;
[0024] FIG. 8 illustrates a block diagram of an excitation circuit
coupled to a rotating field synchronous generator in accordance
with one embodiment of the present disclosure;
[0025] FIG. 9 illustrates an example of a prime mover in accordance
with one embodiment of the present disclosure;
[0026] FIG. 10 illustrates the output voltage induced in the stator
windings relative to the position of the flywheel in accordance
with one embodiment of the present disclosure;
[0027] FIG. 11 illustrates an exemplary flow chart for a method of
predicting a pole slip in a synchronous generator in accordance
with one embodiment of the present disclosure;
[0028] FIG. 12 illustrates another exemplary flow chart for a
method of predicting a pole slip in a synchronous generator in
accordance with a second embodiment of the present disclosure;
[0029] FIG. 13 illustrates another exemplary flow chart for a
method of predicting a pole slip in a synchronous generator in
accordance with a third embodiment of the present disclosure;
and
[0030] FIG. 14 illustrates a block diagram of an excitation circuit
coupled to a rotating field synchronous generator in accordance
with another embodiment of the present disclosure.
DETAILED DESCRIPTION
[0031] As used herein, the following terms have the meanings
ascribed thereto as set forth below. "Logic" may refer to any
single or collection of circuits, integrated circuits, processors,
transistors, memory, combination logic circuit, or the like or any
combination of the above that is capable of providing a desired
operation(s) or function(s). For example, logic may take the form
of one or more processors or microcontrollers executing
instructions from memory, application specific circuits (ASICs),
state machines, programmable logic arrays, integrated circuits,
discrete circuits, etc. that is/are capable of processing data or
information, and any suitable combination(s) thereof.
[0032] With reference to FIGS. 1-3, a conventional rotating
magnetic field synchronous generator 100 is illustrated. Generator
100 includes stator 102 and rotor 104. As is known in the art,
stator 102 is conventionally constructed of insulated metal sheets
that contain grooves with copper windings and rotor 104 is
conventionally an electromagnet made of steel with symmetrically
distributed longitudinal grooves containing excitation windings
302. Stator 102 includes three sets of stator coils or windings 106
that are positioned 120 degrees apart from one another. Those of
skill in the art will recognize that other materials and designs
may be used in the manufacturing and implementation of stator 102
and rotor 104 without deviating from the spirit of this disclosure.
For example, rotor 104 may be a permanent magnet without windings
instead of an electro magnet. FIG. 1 illustrates the rotor magnetic
flux (.PHI.f) at 108 and FIG. 2 illustrates the stator magnetic
flux (.PHI.s) at 110. FIG. 3 illustrates airgap 304 and the
resultant or airgap flux (.PHI.r), at 306. For purposes of this
disclosure, the direction of the rotor 104 is counterclockwise as
shown in FIG. 3.
[0033] FIGS. 3-6 illustrate the rotor 104 in different positions
with respect to stator 102 for the purposes of demonstrating
different load angles (.beta.). FIG. 3 shows the generator 100 at
rest with no load. As such, the load angle (.beta.) is 0 degrees.
FIG. 4 shows the generator 100 in operation in steady state with
the load angle (.beta.) at 45 degrees. FIG. 5 shows the generator
100 in operation with the rotor 104 in such a position that the
load angle (.beta.) is at the edge of the stability zone (i.e., at
110 degrees). FIG. 6 shows the generator 100 in operation with the
rotor 104 well past the stability zone and quickly approaching a
pole slip. In FIG. 6, the load angle (.beta.) is at 270 degrees. As
illustrated in FIGS. 3-6, the resultant or airgap flux (.PHI.r) 306
is slightly distorted while the rotor 104 is in operation within
the stability zone (e.g., in FIG. 4) and is grossly distorted while
the rotor 104 is in operation outside the stability zone (e.g., in
FIGS. 5-6).
[0034] With references to FIGS. 7-10 and 14, the apparatus for pole
slip protection as set forth in this disclosure is described. FIG.
7. illustrates a block diagram 700 of prime mover 702 coupled to a
rotating field synchronous generator 100 in accordance with one
embodiment of the present disclosure. In one embodiment, the prime
mover 702 is an internal combustion engine. In other embodiments,
prime mover 702 may be a gas, steam or water turbine. Other
engines, turbines, and machines may also be used as prime mover
702. During operation, the prime mover 702 rotates the rotor 104
using, for example, an output shaft 704 that is mechanically
coupled to the rotor 104.
[0035] A such, prime mover 704 may include, an internal combustion
engine 900 (FIG. 9) that includes a plurality of pistons 902 that
are linked to a crankshaft 904. The crankshaft 904 is coupled to a
flywheel 906 having a plurality of teeth 908, which engage a gear
(not shown) coupled to output shaft 704. During operation, the
pistons 902 in the internal combustion engine 900 drive the
rotation 910 of the crankshaft 904, which in turn rotates the
flywheel 906. As the flywheel 906 turns, so does output shaft 704
and rotor 104.
[0036] FIG. 8 illustrates a block diagram of an excitation circuit
802 coupled to the rotor 104 through circuit breaker logic 804 and
a rotary electrical interface 806. The excitation circuit 802
provides the dc current 805 for the rotor windings 302 (FIG. 3). In
one embodiment, excitation circuit 802 includes a three phase ac
source 810 coupled to an ac to dc converter 812 such as a thyristor
circuit. One of skill in the art, however, will recognize that
rotor 104 may not require an excitation circuit 802 (i.e., it may
include a permanent magnet instead of an electromagnet).
[0037] The dc current 805 is output to the rotor windings 302 using
a rotary electrical interface 806 such as a set of slip rings
and/or brushes. Other interfaces and/or excitation circuits may
also be used and are contemplated by the present disclosure. As
prime mover 702 rotates rotor 104, the dc current in the rotor
windings 302 creates the rotor magnetic field 108, which in turn
induces the three phase sinusoidal voltages 814 in the stator
windings 106. The induced voltages 814 are coupled to the gird or
distribution system (also known as the "mains").
[0038] The pole slip prediction apparatus includes a pole slip
prediction unit 820 that is coupled to receive stator voltage
frequency signal 822 representative of the voltage output frequency
at the stator windings/terminals 106. Stator voltage frequency
signal 822 is generated by stator voltage frequency detector 818.
In one embodiment, stator voltage frequency detector 818 is
directly connected to busbar 816 and thereby receives the stator
output voltages 814. As is known in the art, busbar 816 is any
suitable set of conductors to which all the generators and feeders
connect within a substation.
[0039] In one embodiment, pole slip prediction unit 820 also
receives a rotor frequency signal 826. Rotor frequency signal 826
represents the speed by which rotor 104 is rotating (e.g., the
number of revolutions per minute). Rotor frequency signal 826 is
generated by a mechanical frequency sensor 828. Mechanical
frequency sensor 828 may be any suitable transducer or sensor that
is capable of measuring the rotational speed of rotor 104.
[0040] The rotor magnetic field 108 rotates at the rotor frequency,
and the stator magnetic field 110 rotates at the stator frequency.
During stable operation of generator 100, the rotation of the rotor
flux (.PHI.f) and the stator flux (.PHI.s) should be locked or
synchronous with a load angle (.beta.) equal to approximately 45
degrees. Accordingly, the ratio of the rotor frequency to the
stator voltage frequency is a constant. If the rotor frequency
increases disproportionately to the stator voltage frequency, then
the rotor 104 and the rotor magnetic field 108 is running faster
than the stator frequency and the stator magnetic field 110 and the
load angle (.beta.) is getting bigger.
[0041] Pole slip prediction unit 820 measures the load angle
(.beta.) using the rotor frequency signal 826 and the stator
voltage frequency signal 822. In particular, the load angle
(.beta.) can be determined by taking the integral over time of the
difference between the rotor frequency and the stator voltage
frequency. That is:
.beta.=.intg.(rotor frequency-stator frequency)dt
[0042] When load angle (.beta.) increases above 90-110 degrees or
any other predetermined value chosen to avoid a pole slip, pole
slip prediction unit 820 sends trip signal 830 to circuit breaker
logic 804. In response to trip signal 830, circuit breaker logic
804 opens the circuit and disconnects the generator 100 from the
grid.
[0043] With reference to FIG. 11, an exemplary flow chart for a
method of predicting a pole slip in a synchronous generator is
illustrated. The method begins at block 1102 where the method is
initialized. The method may be initialized by, for example, using a
synchronous generator such as synchronous generator 100 and
generating output voltage 814 that are induced at the stator
windings 106 as described above with reference to FIGS. 1-6. The
method continues at block 1104, where the frequency of the induced
output voltages (i.e., the stator voltage frequency) is determined.
In operation, this may correspond to using a stator voltage
frequency detector 818 to determine the frequency of the induced
voltages 814. One of skill in the art will recognize that block
1104 corresponds to determining the rotational speed of the stator
flux (.PHI.s).
[0044] The method then proceeds in block 1106 to determining the
rotational speed of the rotor. As described above with reference to
FIGS. 7-8, a mechanical frequency sensor 828 may be employed to
determine the rotational speed of the rotor 104. One of skill in
the art will recognize that block 1106 corresponds to determining
the rotational speed of the rotor flux (.PHI.r) and will further
appreciate that blocks 1104 and 1106 are interchangeable (i.e., may
be performed in reverse order or simultaneously).
[0045] Next, the method determines the load angle (.beta.) in block
1108. In one embodiment, the load angle may be determined using
pole slip prediction unit 820, which may employ suitable logic to
take the integral over time of the difference between the rotor
frequency and the stator frequency as .beta.=.intg.(rotor
frequency-stator voltage frequency)dt.
[0046] At block 1110, the method determines whether the load angle
is greater than a predetermined value. For example, the method may
determine whether the load angle is greater than 90 degrees. In
another example, the method may determine whether the load angle is
greater than 110 degrees. It is contemplated that the predetermined
value may be any suitable value. In general, the predetermined
value is selected as the threshold where, once the load angle is
greater than the predetermined value, it is determined that a pole
slip is inevitable or at least very likely to occur.
[0047] In other examples, the predetermined value may be a
parameter that can be changed or adjusted according to the
characteristics of the synchronous generator or according to the
needs of the user. In one embodiment, prediction unit 820 allows
for the predetermined value to be modified. The modification of the
predetermined value can be caused through a user interface or other
input mechanism by a user or can be modified automatically in
response to historical data regarding the synchronous generator
collected over time.
[0048] If, at decision block 1110, the answer is "no", the load
angle is not greater than the predetermined value and a pole slip
is not expected, then the method returns to block 1104 and the
method continues. If, however, at decision block 1110, the answer
is "yes", the load angle is greater than the predetermined value
and a pole slip is expected, then the method continues at block
1112. There, the method disconnects the generator (e.g., generator
110) from the grid to avoid a pole slip. In one embodiment, pole
slip prediction unit 820 may issue a trip signal 830 to circuit
breaker logic 804 to effectuate the removal of the generator 100
from the grid. Those of skill in the art will recognize that the
generator 100 may alternatively be shut down using other
techniques. The method then concludes at block 1114 where a pole
slip has been predicted and the generator has been shut down or
otherwise disconnected to avoid the severe stresses that a pole
slip would cause to the generator.
[0049] Pole slip prediction unit 820 may, alternatively, issue trip
signal 830 when it observes that the determined load angle (.beta.)
is "trending" forward. In other words, pole slip prediction unit
820 may be configured to not issue the trip signal 830 whenever the
determined load angle (.beta.) over a given period of time is
relatively stable and to issue the trip signal 830 whenever the
determined load angle (.beta.) over time is moving fast. Other
parameters or trends can also be used as thresholds or triggers
upon which trip signal 830 is issued. Examples of thresholds that
can be used in prediction unit 820 include predetermined or given
levels of the rate of change of load angle (.beta.), spikes in the
rate of change of load angle (.beta.), or comparison of load angle
(.beta.) or the rate of change of load angle (.beta.) to historical
or the like or other saved data regarding characteristics of the
synchronous generator or the like. Other thresholds or parameters
known to one of ordinary skill in the art may also be used to
achieve the desired operation(s) or function(s).
[0050] With reference to FIG. 12, another exemplary flow chart for
a method of predicting a pole slip in a synchronous generator is
illustrated. The method of FIG. 12 is identical to that of FIG. 11
with the exception of the decision block. Instead of using decision
block 1110 as described in FIG. 11, FIG. 12 uses decision block
1202. As part of decision block 1202, the method seeks to determine
whether the load angle is trending forward over a predetermined
interval of time. To the extent the answer is "no", i.e., the load
angle is relatively stable over time, the method continues at block
1104. To the extent the answer is "yes", i.e., the load angle is
advancing forward at a rate that exceeds a predetermined value, a
pole slip is determined to be likely and the method continues at
block 1112.
[0051] One of skill in the art will appreciate that the
predetermined rate may be chosen to be any value based on the
tolerances of the system and generator. For example, if the
observed load angles tend to insignificantly fluctuate over time
without causing a pole slip, the predetermined value may be a
relatively larger value. It is appreciated that pole slip
prediction unit 820 may include or be coupled to suitable memory
for storage and retrieval of historical or test data regarding
characteristics of the synchronous generator (or similar
generators) such that prediction unit 820 can determine whether the
determined load angle is trending forward or staying relatively
stable over a suitable interval of time. It is also appreciated
that in the first pass (or first several passes) through the method
disclosed in FIG. 12, decision block 1202 will yield a "no" by
definition as there would be no previously determined load angles
(or too few load angles) with which to compare the current load
angle. Finally, it is also appreciated that any number of load
angles may be used to determine whether the load angle is stable or
trending forward. In one embodiment, the number of load angles used
in decision block 1202 may be a system parameter that corresponds
to the interval of time used to determine whether the load angle is
stable or trending forward.
[0052] In both embodiments described above, the pole slip
prediction unit 820 may measure the load angle (.beta.) in
real-time, i.e., with every period of the stator output voltages
814 (e.g., every 20 ms for 50 Hz countries or every 15 ms for 60 Hz
countries). Those of skill in the art will recognize that the pole
slip prediction unit 820 may determine the load angle (.beta.) at
any other predetermined intervals of time.
[0053] With reference to FIGS. 14 and 10, a third embodiment is
illustrated. Unlike the embodiments described with reference to
FIG. 8, the pole slip prediction apparatus does not include
mechanical frequency sensor 828 or stator voltage frequency
detector 818. Instead, pole slip prediction apparatus includes
count sensor 1406 and stator voltage period detector 1402. Stator
voltage period detector 1402 generates stator voltage period signal
1404 that has a period that matches the period of the stator output
voltages 814. In one embodiment, stator voltage period signal 1404
generates a periodic square wave with a period corresponding to the
period of the stator output voltages 814.
[0054] Count sensor 1406 may be located adjacent to flywheel 906 or
any other gear coupled to output shaft 704 and generates tooth
count signal 1408. Tooth count signal 1408 is, in one embodiment, a
periodic square wave or impulse train where each rising edge of the
square wave or each impulse represents the detection of a new tooth
908 of flywheel 906 or other gear. Count sensor 1406 can be any
suitable transducer. In one embodiment, count sensor 1406 is a
magnetic pick up sensor that generates a magnetic field of a
particular strength and that measures disturbances in the magnetic
field. As the teeth of the flywheel 906 or other gear turn during
normal operation of prime mover 702, the teeth intersect the
magnetic field and create a disturbance that can be observed and
detected by the magnetic pick up sensor. Each rising edge of the
square way or each impulse of the tooth count signal 1408
corresponds to such a disturbance in the magnetic field (or a
tooth) as detected by the count sensor 1406.
[0055] Pole slip prediction unit 820 counts the number of new teeth
"observed" by the count sensor 1406 for each period of the output
voltages 814 using the stator voltage period signal 1404 and the
tooth current signal 1408 and determines the load angle (.beta.)
using the following equation:
.beta.={(# of Teeth Counted) mod (Total # of Teeth)}*p*360/(Total #
of Teeth); where;
[0056] (# of Teeth Counted)=the number of new teeth observed for
each period of the output voltage 814 as determined based on the
tooth count signal 1408 and stator voltage period signal 1404;
[0057] (Total # of Teeth)=the total number of teeth on flywheel 906
or other gear, a predetermined value;
[0058] p=the number of pole pairs (e.g., 1, 2, etc.), a
predetermined value;
[0059] mod=the modulo function; and
[0060] *=the multiplication operator.
[0061] As is known in the art, the expression "a mod b" gives the
remainder of the division of "a" by "b."
[0062] With reference to FIG. 10, an exemplary flywheel with 16
teeth is illustrated with a "marked" tooth to show the relative
rotation of the flywheel over two periods of output ac current 814.
For purposes of example, the number of pole pairs "p"=1. At the
conclusion of the first period of the stator voltage period signal
1404, i.e., at t2, the flywheel has completed 1 full rotation. As
such, the load angle (.beta.)=
.beta.=(16 mod 16)*1*360/16;
[0063] and .beta.=0 degrees.
[0064] At the conclusion of the second period of stator voltage
period signal 1404, at t3, the flywheel has made more than one full
rotation and has counted 18 teeth per voltage period. As such, the
load angle (.beta.)=
.beta.=(18 mod 16)*1*360/16;
[0065] .beta.=45 degrees.
[0066] In one example, pole slip prediction unit 822 issues trip
signal 830 whenever load angle (.beta.) is greater than 90-110
degrees or any other predetermined value selected to correspond to
an inevitable pole slip.
[0067] Pole slip prediction unit 822 may also perform a second
calculation to determine whether the rotor 104 (and hence flywheel
906 or other gear) has made more than 1 rotation. In other words,
with reference to the above example, the (# of Teeth Counted)=17 or
34, the above equation and the use of the "mod" function in
particular has no way of determining whether the rotor 104 has made
just over 1 full rotation or just over 2 full rotations for each
period of the output voltages 814 (i.e., the period of the stator
voltage period signal 1404). If, for example, (# of Teeth
Counted)=34, the equation above will give a false positive when in
fact a pole slip has already occurred. To compensate for the
foregoing, protection unit 822 may simultaneously perform a second
equation to identify false positives:
.beta.2=[(# of Teeth Counted)-{1.25*(Total # of
Teeth)}]*p*360/(Total # of Teeth).
[0068] Under this embodiment, if any determined (.beta.) (i.e.,
(.beta.) or (.beta.2)) is greater than 90-110 degrees, pole slip
prediction unit 820 generates trip signal 830. For example, if
count sensor 832 counts 34 teeth:
.beta.=(34 mod 16)*1*360/16
[0069] .beta.=45 degrees.
.beta.2=[34-{1.25*16}]*1*360/16;
[0070] .beta.2=315 degrees.
[0071] In such an instance, .beta.2 is well above 90-110 degrees
and a trip signal 830 is generated to disconnect the generator 100
from the grid.
[0072] FIG. 13 illustrates another exemplary flow chart for a
method of predicting a pole slip in a synchronous generator. The
method includes initialization block 1102 as described above and
continues with block 1301 where the stator voltage period is
determined. In one embodiment, the stator voltage period is
determined by the stator voltage period detector 1402, which
generates a stator voltage period signal 1404. The method continues
with block 1302 where the number of gear teeth (e.g., teeth 908 of
flywheel 906 mechanically coupled to the rotor 104) is counted. In
one embodiment, the number of gear teeth is counted by count sensor
1406, which generates tooth count signal 1408.
[0073] The method continues with block 1304 where the load angle is
determined based on the stator voltage period and the number of
gear teeth counted. In one embodiment, pole slip prediction unit
820 receives the stator voltage period signal 1404 and the teeth
count signal 1408 and determines the load angle based on the
following equation: .beta.={(# of Teeth Counted) mod (Total # of
Teeth)}*p*360/(Total # of Teeth) with variables defined above.
[0074] The method then continues with decision block 1110. If the
answer at block 1110 is "yes", then the method proceeds with blocks
1112 and 1114 as described above. If the answer at block 1110 is
"no" then the method may either return to block 1104 or 1302, or
alternatively, the method may include an optional decision block
1308 which determines if the decision at block 1110 yielded a false
positive. In other words, block 1308 seeks to determine whether the
rotor 104 has undergone an extra revolution during the course of a
single period of the stator output voltage 814 such that a pole
slip has already occurred. In one embodiment, pole slip prediction
unit 820 determines whether the rotor has undergone an extra
revolution using the (.beta.2) equation described above.
[0075] If the answer to decision block 1308 is "yes", a pole slip
has already occurred and the method continues with blocks 1112 and
1114. If the answer to decision block 1308 is "no", a pole slip has
definitively not occurred and the method continues with blocks 1104
or 1302.
[0076] One having skill in the art will appreciate that each of
excitation circuit 802, circuit breaker 804, stator voltage
frequency detector 818, pole slip prediction unit 820, mechanical
frequency sensor 828, count sensor 1406 and stator voltage period
detector 1402 in the foregoing FIGs. may include or otherwise be
comprised of logic.
[0077] Among other advantages, the above pole slip prediction
apparatus and method for making and using the same efficiently
predicts when a pole slips will inevitably occur in a generator
before the generator is exposed to tremendous stress. In addition,
the pole slip prediction apparatus and method is less expensive and
complicated to implement than conventional impedance methods and
can therefore be readily implemented to prolong the life of
generators.
[0078] Other advantages will be recognized by one of ordinary skill
in the art. It will also be recognized that the above description
describes mere examples and that other embodiments are envisioned
and covered by this disclosure. It is therefore contemplated that
the present invention cover any and all modifications, variations
or equivalents that fall within the spirit and scope of the basic
underlying principles disclosed above and claimed herein.
* * * * *